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1 Zoology, La Trobe University, Bundoora, Victoria, Australia
* To whom correspondence should be addressed. E-mail: t.dutka{at}latrobe.edu.au.
The Na+-K+ pumps in the transverse tubular (T-) system of a muscle fiber play a vital role keeping [K+] in the T-system sufficiently low during activity so as to prevent chronic depolarization and consequent loss of excitability. These Na+-K+ pumps are located in the triad junction, the key transduction zone controlling excitation-contraction (EC-) coupling, a region rich in glycolytic enzymes and likely having high localised ATP usage and limited substrate diffusion. This study examined whether Na+-K+ pump function was dependent on ATP derived via the glycolytic pathway locally within the triad region. Single fibers from rat fast-twitch muscle were mechanically skinned, sealing off the T-system but retaining normal EC-coupling. Intracellular composition was set by the bathing solution, and action potentials (APs) triggered in the T-system, eliciting intracellular Ca2+ release and twitch and tetanic force responses. Conditions were selected such that increased Na+-K+ pump function could be detected from the consequent increase in T-system polarization and resultant faster rate of AP repriming. Na+-K+ pump function was not adequately supported by maintaining cytoplasmic [ATP] at its normal resting level (~8 mM), even with 10 mM or 40 mM creatine phosphate (CP) present. Addition of as little as 1 mM of phospho(enol)pyruvate (PEP) resulted in a marked increase in Na+-K+ pump function, supported by endogenous pyruvate kinase (PK) bound within the triad. These results demonstrate that the triad junction is a highly restricted micro-environment, where glycolytic re-synthesis of ATP is critical to meet high demand of the Na+-K+ pump and maintain muscle excitability.
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